Which royal dynasty did Leonidas I represent as king of ancient Sparta?

Leonidas I represented the Agiad dynasty as king of ancient Sparta.

Sparta traditionally had two hereditary royal houses: the Agiads and the Eurypontids. Each house supplied one of Sparta’s two simultaneous kings, a distinctive feature of the Spartan political system. Leonidas belonged to the older Agiad line.

Leonidas became famous for leading the Greek force at Thermopylae in 480 BCE during the invasion of Greece by the Persian king Xerxes I. His death there helped make him one of the best-known figures in Spartan history, although the wider defense included soldiers from several Greek states.

The Agiad dynasty is sometimes confused with the Eurypontid dynasty, which supplied Sparta’s other king at the same time. Leonidas’s half-brother Cleomenes I was also an Agiad king, while his successor and son-in-law, Pleistarchus, continued the Agiad line. The two-dynasty arrangement endured for centuries, even as Sparta’s institutions and military influence changed.